Process of making silicon carbide articles and raw batch therefor



June 11, 1935. R. c. BENNER ET AL 2,004,595

PROCESS OF MAKING SILICON CARBIDE ARTICLES AND RAW BATCH THEREFOR Filed Aug. 26, 1932 o I700 C- I6oo ALUMlNA-FLUORSPAR I4oo Iao

8'0 60 4'0 2'0 a AL O3 PERCENTAGE I N LIQUID I I600C- 2, I |aooc- 3. MAGNESIA-FLUOR$PAR 700 FORSTERITE-DIOPSIDE 1400" Ieoo I300 I5oo I200 I4oo fi- I300 I I 4 M 90 80 70 50 2M 9Q 80 60 40 20 I600" SILIOA-MAGNESIOFERRITE I5oo o INVENTORS I400 RAYMOND C; BENNER HENRY N. BAUMANN JR. BY m d/ I ATTORNEY SiO SO 8O 60 50 4O 30 55 ficiently soft to Patented June 11, I935 PROCESS ARTICLES 7 Raymond C.-=B enner;an

Niagara Falls, N, Y borundum 'Compan corporations Application August- In. Great Brita r ,1 MAKING Y 's'ILiooN AN RAW BATCH-T31 I assignorsI to T 26, 1932, Serial N in November 20,

fiARBIDE P EFQR, Henry- N; Baum he Car- Falls,j.N.'Y.,ia1 a Niagara,- I

' g4c1 ims, (ores-1 5 .1

This invention relates to ceramic articles and compositions-,-. and to a bondther t or composition out damage w temperature. The present .2111 tutesa continuation in part of cations Serial 'No. 268,364 filed 10 Serial No. 349,739 filed March Our invention is. particularl refractory in whcihthe chief i carbide but may be applied wi stantially higher plication consti- 25, 1929. y applicable-to a ngreclien'tis silicon th but'slight modisubsequently at temper 25 above the said burni weakening or other .ad

atures' 300 C.- or more ng temperature :with

I out verse effect. Our refracn of refractories in accordtion, we util For the productio ance with our inven class of m 40 softening ture between the remaining mor progressively in th perature is raised? above". the initi point, thereby co e refractory refractory is subje progresses over t A mixture of th terized by the pre is raised to and J' of the most fusi given time) of a cted. is raised an das solution he desired range sence in it (as the tem ust'beyond the softeni ble constituent presen small amount of mat exert a ,bonding action on the efor, and parportion ofthe hon melting point of t softening" or in given compositio Changes in crystal sidered in this con CL; and: in which portionodissolves esoftened portion as the tem--' a1 softening e class which we use is'charac-* perature ng point t at any erial suf- In the first case (ti 7 d is immediately absorhed hy m We that best results are secured when the hBfiJLSBd bOITlQiOII is increased by from 5 to C."(approximatelylfforeach refractory component there- Itwill be note specification we refer sometimes ndl sometimes to, Our meaning in the variou the immediate contxti either case is that atjwhichla liquid, having a. n [is fingequ'ilibrium with the co'nstituent' of I the bond. structure, etc. are not connect'ion.

of "b ds of the he temperat'ur e in solid refractory 7 Several e V f.',[ which'are commonly production of refractories from relaf non-plastic materials of thetype in of carbonaopen and permeuseclin'the tively inert o ;;be successfully, a should remain rial; that it is to say, at temperatures below 900 to 1000 C., as the softening of thefbond serves to seal the refractory against, permeation by In order to reduce the expense of burninghowever. it is--desirable that theoxidi'zing gases.

bond start to'matureat a =temperatureas little above this range as is practical. This is also desirable in silicon carbide refractories because H of the fact that silicon carbidemay be oxidized rapidly at temperatures abovel000fffC and. the

softening of the bond coat's'the siliconfcarbide thus excluding oxidizing gases; Duezto the: pres ence of various impurities, such as ironoxide, in ordinary commercial grains, 'thef'fusion point of the added bonding materials is usuallyre duced a considerable amount'belowthe fusion point of the bond in the absence of suchimpuri' sive burning temperatures may. not-be required we prefer to select these bonds: having an initial softening point not greater than 1f100 C ,It is moreover desirablethatthe melting pointof the combined bond ingredients; increase continuously in the manner notedovera rangeof atf least 300? 0. above" the initial .softening'point tures falling within the class of :bonds'conteme" the liquid) 'in af'system composedof alumina and range" of possible 'application continuously matur- Certain "bonds which are 'in g also are resistant to reducingconditions; in an unusual degree. Such bonds are substantially silicafree and include those containing a considerable. proportion of an alkaline earth halide" such as calcium fluoride, This classof bonds is claimed in our copending application Serial No.,589,527 filed. January]2,8,11932 as a continuation of our application Serial..;No. zeactsjfiiea Apriii 1923.; 1

Wehave found that-there arenumerous mix plated; by our invention .from which. ;we have selected those below for' purpose of illustration. The melting or freezin'g points-pf these representative mixes or systems areshown in the accompanying figures in which:

- Figure 1 represents the. variation of softening point vs. percentage composition of alumina (in fiuorspar; V A Y Figure showssimilar values for the system comprising magnesia'andfluorspar; i

FigureB is a similar curve fora systemwhere in both components-are silicates; namely. forsterite (MgiSiQi) and diopside l(CaMg(SiOa) 2);

' Figure 4 is a similar. curve for a binarysystem r v 5 parts byweight :of ia'mixture of "equal parts "of of silica and magnlasioierrite. q

With reference to Figure 1,.-it willgbe': seen that the curve showing melting point vs. -.com-1 position in the alumina-fluorspar. system's as-T cends continuously with increasing alumina itself-is very close to the calcium fluoride line,

and melts only 50 below the melting point of calcium fluoride. ,It isfthus apparent that the desired'co ndition of increasing fusion point withing 100% alumina;

In Figu e order that the re fractory may a have :ar.wide

of bond, and-the.:re'sulting-:refractories do not 2 there is illustrated? binary as; 75

Simple binary systems containing silica as one of theirg components cannot generally be "adapted tc this principle on account of the ir- -regularity 'of their melting point curves.- We have foundfjhowev'er, that our invention is applicable to morefcpmplex systems such as those in which the components themselves are silic-ates. Anexample of such a combination is the system Diopside-Forsterite (CaMg(SiOs)2-Mg2SiO4) shown in' Figure 3. The fusion point in this system between the range 15% and 100% For- .steriterises gradually but continuously from 1375 C. to 1890$CL (7 perpercent). In practice .we may use-eitherthe natural oii synthetic mixtures having .the .Isame compositions.

Figure 4- relating to a part'of tlfe-system .silica-magnesioferrite shows a continuously" rising ,meltingspoint from that of -the eutectic up to the fUSlOIYpOlIltFlOf -silica"'(4.9 -pe'r percent)".

This system-. furni'shes"a very valuable bond for silicon carbide refractories .in which part of the silica. which; causes the risein fusion point of the bond,-..resu-lts from oxidation of the surface ofjthe silicon carbide grain. below' the eutectic fusion-temperature .of the system: -Af ter-the melting point :of the eutectic composition is reached, this silica is 7 dissolved continuously in the. magnesioferrite-silica. melt'as the temperature is increased"; This bond is in contact'with and lenvelopesthe silicon carbide grains to an unusual degree, thus retarding further-oxidation ofthesilicon carbide. I ,In a-system where thesoftening point of the most fusible component: is below approximately 1150? C., it is possible to first calcine the ingredient5;at;1l50 CL: to complete suchsolution as occurs up to thisfpoint, then pulverize-the result A ing fused product and-thereafterus'e itin abond.

A system comprising silica and sodium silicate presents such a condition; although such a system is not. especially desirable'for mostjo'perations because; the melting point increases too rapidly for each percent ofincrea'se in the silica that goes intosolutionj I As an example of the application of our invention, we cite "the following illustrative case. A raw batch; is .-prepared comprising 90' parts by weight of 14 and .finer silicon carbide'and l0 thoroughlya dry} mixed, afterwhich sufficient water. is added to "temper the' mix suitably for fabrication. -jgArticles. of the desired shape and size are pressed-aor tamped from this mix and: then dried in the-usual :way and fired in a kiln at a temperature of 'from 1300- 0. to 1400 C.

Refractories ofthis composition have proved very satisfactory in service; .Wehave found that on account of the .peculiar properties of increasing percentage ofTalumina prevails over; a wide range from a composition containing; only'approximate 107% to'a'composition contain- V bondsof our type, it ispossible to vary the proportion of bondand grain considerablyi For example, the, mix may'vary at le'ast from 5 to 15% vary much in character.

" organic binder' such as dextrin, 'moiste'ned and As an example .of a magnesioferrite silicon carbide refractory, we use a dry mix of .043 cal/cm /sec/C.,

bonded the following composition: V

Parts Silicon carbide .(14 mesh and finer refractory grain) '90 An intimate mixture of calcined magnesite and commercial hematiteintheir molecularratio. (All finely pulverized) 10 This composition is mixed with a temporary either pressed or tamped into the desired refractory shapes. Commercial hematite contains silica only as an appreciable impurity, and "this silica initiates the continuouslymaturing proc- "ess, the-rest of the silica'coming'from theoxidation of the surface' of] the silicon carbide grains with which the bond is thoroughly mixed. The refractory, after drying, is burned to at least 1400 C. and preferably higher. 'Silicon carbide refractories made from this composition were'found 'tohave a thermal conductivity of a value approximately 10% higher than that for any other bonded silicon carbide refractory with which we are familiar.

In modifying the first above mixture for use with fused alumina,'we may either simply substitute fused alumina for the silicon carbide or we may use a mix of the followingtype Parts Fused alumina 14 and finer Calcined alumina and finer 10 A mixture of equal parts of calcium fluoride and calcined alumina 5 Temporary binder 2 The calcined alumina is less inert chemically than fused alumina in the same grit sizes, and its use results in a more rapid and more thorough bonding action than when fused alumina alone is used.

We believe that the excellent resistance which our materials show in respect to spalling is due to the fact that they at no time contain a large amount of glassy material, such as is present in refractories which are burned to a temperature which softens the entire bond leaving large amounts of highly vitrified material in the ware. In our refractories no highly vitrified material is present. In cases where the bond comprises alumina and a fiux the alumina crystallizes out of the molten bond readily as the refractory is cooled, leaving a refractory bonded with alumina crystals and the more fusible eutectic. In cases where the bond is a silicate, crystallization is slower and a small proportion of semi-vitreous bond is left as the refractory cools. This semivitreous constituent is small in amount and has a fusion point approximately equal to the highest temperature to which the brick has been fired. It is present in an amount just suificient to effect proper bonding without imparting the glassy structure which is so subject to damage by spalling. It has been found that upon '30 quenchings according to the A. S. T. M. (American Society for Testing Materials) standard spalling tests as described on page 209 of their Tentative Standards-1924, a fused alumina brick made from the mix mentioned above will lose on the average less than 20% of its original dry weight in thirty quenchings, whereas the fused alumina refractories on the market prior to this invention showed only about one-fifth of this resistancejospalling.

' sufiicient to burn Upon use at any-temperature below theorig'inal burning temperature "our refractories-- are entirely unaltered by heat? If used attemperatures' higher than the burning temperature, the

" bond continiuesto "maturein the same manner as described above without damage to the-refractory. v v w Ceramic brick or shapes of our manufacture show practically no volume' change whether fired-at 1300 or 15 00- -"C. '-The porosity when fired 'at-1300 will differ by less than"1%* from its porosity when fired at' 150O C1 4 v While for-the sake of simplicity we have illustrated the bonds" inthe' foregoing illustrations as 'binary' compounds, our invention also contemplates the use of bonds having a larger num; berof components than two, always provided however that oneof the bond components of a highly refractory nature is present in'excess of the remainder of the mixture; Thisfref'ractory component must moreoverbe' soluble in the remainder of the mixture in increasing amounts as the temperature increases over the specified range "in the manner'fexplaine'd' abov'et-The magnesioferrite silica system is in reality an illustration of the use of such a multi-compo- 1 nent bond, the ratio of magnesia to iron oxide being variable over a considerable range so long as the silica is present in sufiicient excess to yield the desired increasing melting point curve. The diopside-forsterite system is also any illustration of the presence of more than two components (i. e. lime, magnesia and silica) under special conditions.

In addition to use in 'prebumed shapedrefractories, our invention is also'applicable to refractory cements such as those which are rammedv in and burned in place or those used as mortar. for laying up refractoryfurnace linings, etc." In fact the value of our invention is'particu larly great in such applications because our cement can besatisfactorily vitrified in place at a relatively lower temperature than is required for many mixtures of comparable refractoriness,

.and yet is not harmed even by burning at excessive temperatures for considerable periods.

Having thus described our invention, in a full, clear and concise manner, what we claim 1. In the process of making silicon carbide refractory articles, the steps which comprise admixing with silicon carbide grain a carbonaceous temporary binder anda vitrifiable binder of that class of mixtures of fluxing and refractory materials, each of which mixtures is characteriz'ed when present in such' an admixture by initial softening of a portion only thereof at but not substantially below a temperature ap-' proximating that at which rapid oxidation-of silicon carbide begins, in which softened portion the more refractory portion dissolves in- .in'creasing amounts progressively over a range of at least 400 C. as the temperature is raised above the initial softening point, forming the mixture, heating the formed mixture for a time out the temporary binder at a temperature below that at which rapid oxidation of silicon carbide begins, and thereafter raising the temperature to a point sufficient to *cause the bond to flow enough to protect the silicon carbide from oxidation which would otherwise takeplace.

2. In the process of making silicon carbide refractories, the steps which comprise admixing withosilicon carbide grain and a. carbonaceous 15 Mo a bide: begins, thersilicon carbide being protected against oxidation at such; higher temperatures terial at a temperature,beiowx the-initial ma turing point of. the binder', -and thereafter heating the formed.- mixture to a temperature above that at which rapid oxidation of silicon carby the continuously maturing, binder.-- L

3. A rawbatch for the anufacturefof siiicon 20 carbide refractories, rs aidiIbatch conrprisingsilicon .carbide.;grains, ,a. carbonaceous temporary binder and a permanent binderwhich comprises a fluxing component which soitens above the temperature required to. burn} out'the carbonaceous binder and beiow the; temperature at which ira'fpidfoxidation of siiicon carbide begins, and. a

"carbide! refractory component soluble in said fiuxing component in continuously and gradually increasing amounts as the temperature .is increased .above thatpoint for at-1east-400:C.L 2

4. In the manufacture:ofisiliconcarbide re fractory articles the steps which comprise preparing arbond' bymixingtogether a;- fiuxing ma terial vand a more refractory material soluble therein; in increasing 'amountswwith increasing temperaturerwherein the most I fusible constit- '-;uent,.present' in'themixture' melts below 1150 0., calcining the mixture to. a temperature approximately 11502ii(}i.'..ar id thereafter pulyerizing the bond'and admi'xingit' withksilicon carbide grain, and a carbonaceous binder; ,formin'if an article from said burning the article to re- ,movejthe' carbonaceous material at aQtmperature below' thatfatjwhich the" .bond .',beg insl.to soften anmqrte emovali, ot the carbonaceous 'ma al bu' sni 's .7 w v w peratufre jsuificient. to cause the bond to 7 develop thus preventing rapidf oxidationjof the silicon -i '1 immerse; arms: p 1WHTENRY imBAUM A; JRH 

